1. Field of the Invention
This invention relates generally to a method for classifying a road surface condition and, more particularly, to a method for estimating the maximum tire/road surface coefficient of friction to determine if the road surface is icy, snowy or neither, where the method includes inducing a specified amount of vehicle acceleration or deceleration.
2. Discussion of the Related Art
The performance of various vehicle control systems, such as collision avoidance systems, adaptive cruise control systems, automated lane-keeping systems, automated braking systems (ABS), turning control systems (TCS), stability control systems, etc., can be greatly improved by providing an estimate of the maximum tire/road surface coefficient of friction. For example, the gains and the commanded values of the vehicle control system can be made adaptive to the maximum tire/road surface coefficient of friction to increase the performance of the system.
The wheel slip of a vehicle wheel during acceleration is defined as the difference in wheel speed between a driven wheel and a non-driven wheel. The wheel slip ratio is the ratio of the difference between the speeds of the driven wheel and the non-driven wheel to the speed of the driven wheel. It can be shown that the slope of the wheel slip ratio road surface coefficient of friction curves for small slip ratios (in the linear region) is almost the same for all road surfaces. FIG. 1 is a graph with wheel slip ratio on the horizontal axis and coefficient of friction on the vertical axis showing wheel slip curves for different road conditions that illustrates this point. Graph line 42 is a wheel slip ratio curve for a dry road condition, graph line 44 is a wheel slip ratio curve for a wet road condition, graph line 46 is a wheel slip ratio curve for a snowy road condition and graph line 48 is a wheel slip ratio curve for an icy road condition.
It is typically difficult to classify the road surface condition when the wheel slip ratio is in the linear operating range. As the wheel slip ratio increases, the slope for the different road surface conditions starts to decrease and become different. However, once the vehicle region of operation is in the non-linear region, it may be too late for the control system to utilize the wheel slip information and adapt to the current road surface conditions. Therefore it is desirable to estimate the maximum tire/road surface coefficient of friction before the wheel slip ratio reaches the value that corresponds to the maximum coefficient of friction. By estimating the slope of the curve at higher wheel slip ratios, it is possible to classify the road surfaces as, for example, icy, snowy, wet or dry.